Arabinogalactan-proteins (AGPs) are proteoglycans with a high level of galactose and arabinose. Their current functions in plant development remain speculative. In this study, (beta-D-glucosyl)3 Yariv phenyl-glycoside [(beta-D-Glc)3] was used to perturb AGPs at the plasmalemma-cell wall interface in order to understand their functional significance in cell wall assembly during pollen tube growth. Lily (Lilium longiflorum Thunb.) pollen tubes, in which AGPs are deposited at the tip, were used as a model. Yariv phenylglycoside destabilizes the normal intercalation of new cell wall subunits, while exocytosis of the secretory vesicles still occurs. The accumulated components at the tip are segregated between fibrillar areas of homogalacturonans and translucent domains containing callose and AGPs. We propose that the formation of AGP/(beta-D-Glc)3 complexes is responsible for the lack of proper cell wall assembly. Pectin accumulation and callose synthesis at the tip may also change the molecular architecture of the cell wall and explain the lack of proper cell wall assembly. The data confirm the importance of AGPs in pollen tube growth and emphasize their role in the deposition of cell wall subunits within the previously synthesized cell wall.

The compatible response in flowering plants is a complex process involving a series of cell communication events leading the pollen tube through the gynoecium to the ovule. We provide the first description of pollen tube growth in vivo in Arabidopsis thaliana. The structure and composition of the transmitting tissue of the septum are examined at the light microscope and transmission electron microscope levels. We demonstrate that once pollen tubes leave the extracellular matrix (ECM) of the central septum, they adhere to and travel along the ECM of the septum epidermis. This ECM, which surrounds the pollen tubes, is secreted through breaks in the cuticle of the septum surface. The transmitting tract ECM stains strongly for acidic polysaccharides. Monoclonal antibodies to esterified and low-esterified homogalacturonans (pectins) do not localize to this ECM. Yariv phenylglycoside and monoclonal antibodies to arabinogalactan proteins (AGPs) provide evidence that AGPs are a component of the septum epidermal ECM.& k w d : Key words Arabidopsis thaliana · Arabinogalactan proteins · High-pressure freezing/freeze substitution · Extracellular matrices · Pollination& b d y :

SummaryCytoplasmic calcium concentration ([Ca 2+ ] i ) and extracellular calcium (Ca

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